This invention relates to the use of mixtures of special diene copolymers and phenol-terminated polyurethanes or polyureas or polyimides in admixture with epoxy resins and/or adducts of epoxy resins with diene copolymers and/or the polyurethane or the polyurea as high-impact epoxy resin adhesives with particularly good low-temperature properties and to reactive, preferably one-component hotmelt adhesives with good low-temperature impact strength.
Reactive epoxy-based hotmelt adhesives are known. In machine and vehicle construction and especially in the construction of aircraft, railway vehicles and motor vehicles, components of various metals and/or composite materials are increasingly being joined together with the aid of adhesives. Epoxy adhesives are widely used for high-strength structural bonding, more particularly as heat-curing one-component adhesives which, in many cases, are also formulated as reactive hotmelts. Reactive hotmelts are adhesives which are solid at room temperature and which soften and behave like a thermosplastic material at temperatures of up to about 80 to 90xc2x0 C. It is only at relatively high temperatures of about 100xc2x0 C. and higher that the latent hardeners present in these hotmelt adhesives are thermally activated so that irreversible curing to a thermoset occurs. To join the components together, for example in the vehicle industry, the adhesive is first applied warm to at least one substrate surface, after which the parts to be joined are then fitted together. The adhesive then solidifies on cooling and, through this physical solidification, establishes adequate handling resistance, i.e. a temporary bond. The parts thus joined together are further treated in various washing, phosphating and dip painting baths. It is only after this that the adhesive is cured at relatively high temperatures in an oven.
Conventional adhesives and hotmelt adhesives based on epoxy resins are hard and brittle in the cured state. Although the bonds obtained with them are generally characterized by very high tensile shear strength, the adhesives flake off under peel, impact or impact/peel stress, particularly at relatively low temperatures, so that loss of bond strength readily occurs when the adhesive joint is subjected to that kind of stress. Accordingly, numerous proposals have already been put forward with a view to so modifying epoxy resins by flexible additives that their brittleness is clearly reduced. One known process is based on the use of special rubber/epoxy resin adducts which are incorporated as heterodisperse phase in the epoxy resin matrix so that the epoxies become more impact-resistant. These epoxy resin compositions are also referred to as xe2x80x9ctoughenedxe2x80x9d. Another known modification of epoxy resins of the above-mentioned type consists in the reaction of a carboxyl-terminated polybutadiene-co-acrylonitrile copolymer with an epoxy resin. This rubber/epoxy adduct is then dispersed in one or more different epoxy resins. The reaction of the epoxy resin with the carboxyl-containing butadiene/acrylonitrile rubber has to be conducted in such a way that the adduct is not prematurely cured. Although correspondingly modified epoxy resin compositions already represent a clear improvement over unmodified epoxy resins in relation to their impact strength, their behaviour under peel or impact/peel stress is still not satisfactory.
EP-A-0 343 676 describes adhesive compositions made up of a mixture of several epoxy resins, a phenolic resin and a polyurethane/epoxy adduct. The polyurethane/epoxy adduct present therein consists of a reaction product of several polyalkylene glycol homopolymers and copolymers containing primary and secondary OH groups, a diisocyanate and at least one epoxy resin. According to the document in question, these hotmelt adhesive compositions show improved shear resistance, peel strength and impact strength in relation to various commercial one-component hotmelt adhesive compositions. Unfortunately, there is no reference to the adhesive properties of the cured adhesive joint at low temperatures.
U.S. Pat. No. 5,290,857 describes an epoxy resin adhesive composition containing an epoxy resin and a powder-form core/shell polymer and a heat-activatable hardener for the epoxy resin. The powder-form core/shell polymer is composed of a core containing an acrylate or methacrylate copolymer with a glass transition temperature of xe2x88x9230xc2x0 C. or lower and a shell containing an acrylate or methacrylate polymer which contains crosslinking monomer units and which has a glass transition temperature of 70xc2x0 C. or higher, the ratio by weight of the core to the shell being between 10:1 and 1:4. These compositions are said to have excellent adhesive properties, such as peel strength, tensile shear strength and T-peel strength, and also good partial gellability. No mention is made of the properties of bonds with these adhesives at low temperatures.
Similarly, U.S. Pat. No. 5,686,509 describes an adhesion-strengthening composition for epoxy resins consisting of powder-form copolymer particles ionically crosslinked with a mono- or divalent metal cation. The core of the core/shell polymer is composed of a diene monomer and optionally crosslinking monomer units and has a glass transition temperature of xe2x88x9230xc2x0 C. or lower. The shell copolymer has a glass transition temperature of at least 70xc2x0 C. and is made up of acrylate or methacrylate monomer units and radically polymerizable unsaturated carboxylic acid units. The adhesive composition is said to contain 15 to 60 parts of by weight of the adhesion-strengthening copolymer powder and 3 to 30 parts by weight of a heat-activatable hardening agent to 100 parts of epoxy resin. These compositions are recommended for use as structural adhesives for automobile parts. No mention is made of the low temperature properties of corresponding bonds.
EP-A-0 308 664 describes epoxy resin compositions which contain an epoxide adduct of a carboxyl-containing copolymer based on butadiene/acrylonitrile or similar butadiene copolymers and a reaction product of an elastomeric isocyanate-terminated prepolymer soluble or dispersible in epoxy resins with a polyphenol or aminophenol and subsequent reaction of this adduct with an epoxy resin. In addition, these compositions may contain one or more epoxy resins. Furthermore, aminofunctional hardeners, polyaminoamides, polyphenols, polycarboxylic acids and their anhydrides or catalytic hardeners and optionally accelerators are proposed for hardening these compositions. The compositions in question are said to be suitable as adhesives which can have high strength, a high glass transition temperature, high peel strength, high impact strength or high tear propagation resistance according to their particular composition.
EP-A-0 308 664 does not indicate whether the compositions described therein are suitable for adhesives with good low-temperature impact strength.
Similarly, EP-A-0 353 190 describes epoxy resin compositions containing an adduct of an epoxy resin and a carboxylated butadiene/acrylonitrile copolymer and a reaction product of a hydroxyl-, mercapto- or amino-terminated polyalkylene glycol with a phenol carboxylic acid with subsequent reaction of the phenolic group with an epoxy resin. According to EP-A-0 353 190, these compositions are suitable for the production of adhesives, adhesive films, patches, sealing compounds, paints or matrix resins. There is no indication of whether the adhesives thus produced have good low-temperature impact strength.
According to the teaching of EP-A-0 354 498 or EP-A-0 591 307, reactive hotmelt adhesive compositions can be produced from a resin component, at least one heat-activatable latent hardener for the resin component and optionally accelerators, fillers, thixotropicizing agents and other typical additives, the resin component being obtainable by the reaction of an epoxy resin solid at room temperature and an epoxy resin liquid at room temperature with one or more linear or branched amino-terminated polyoxypropylenes. The epoxy resins are said to be used in such a quantity, based on the amino-terminated polyoxypropylene, that an excess of epoxy groups, based on the amino groups, is guaranteed. These adhesive compositions have a high peel resistance in the T-peel test which they retain even at low temperatures.
The problem addressed by the present invention was further to improve reactive adhesives of the type mentioned at the beginning to the extent that they would have adequate flexibility and increased peel strength not only at room temperature but alsoxe2x80x94and in particularxe2x80x94at low temperatures (below 0xc2x0 C.). In particular, they would show high peel strength at low temperatures and under impact so that, even in the event of a crash, structurally bonded parts would meet modern safety standards in vehicle construction. These improvements would be obtained without any deterioration in peel strength at high temperatures or in tensile shear strength. In addition, the reactive adhesives would have to exhibit adequate wash-out resistance immediately after application and before final curing. To that end, the adhesive compositions would have to lend themselves as hotmelts to formulation as a highly viscous adhesive suitable for warm application. Another possibility would be to formulate the compositions as an adhesive that could be gelled by a thermal preliminary reaction in a so-called xe2x80x9cwhite body ovenxe2x80x9d or by induction heating of the joined parts.